This invention relates to wireless communication systems, and more particularly to a method for defining the rate at which mobile stations search for pilot signals.
The use of wireless communication systems is growing with users now numbering well into the millions. One of the popular wireless communications systems is the cellular telephone, having a mobile station (or handset) and a base station. Cellular telephones allow a user to talk over the telephone without having to remain in a fixed location. This allows users to, for example, move freely about the community while talking on the phone.
Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication system as described in TIA/EIA, IS-95, Mobile station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, published July 1993. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. In CDMA systems, signals can be and are received in the presence of high levels of interference. The practical limit of signal reception depends on the channel conditions, but CDMA reception in the system described in the aforementioned IS-95 Standard can take place in the presence of interference that is 18 dB larger than the signal for a static channel. Typically the system operates with a lower level of interference and dynamic channel conditions.
A CDMA base station communicates with a mobile station with a signal having a basic data rate of 9600 bits/s. The signal is then spread to a transmitted bit rate, or chip rate, of 1.2288 MHz. Spreading applies digital codes to the data bits, which increase the data rate while adding redundancy to the CDMA system. The chips of all the users in that cell are then added to form a composite digital signal. The composite digital signal is then transmitted using a form of quadrature phase shift keying (QPSK) modulation that has been filtered to limit the bandwidth of the signal.
Base stations inform mobile stations that a call has been placed to it over a forward channel referred to as a paging channel. However, before the mobile station can monitor the paging channel, it must first determine from which base station it will monitor the paging channel. The mobile station preferably monitors the base station that provides the strongest channel signal.
To set up communications with the mobile stations, each base station continuously transmits a pilot channel signal. The pilot signal transmitted by each base station has the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. These pilot signals are detected by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. To ensure flexibility, mobile stations monitor the pilot signal from multiple base stations. This allows the mobile station to be ready to communicate with a number of base stations.
The present invention is a method to measure the desired processing power for mobile stations to monitor pilot signals of multiple base stations. The amount of processing power available to a mobile station is limited. To reduce costs, manufactures attempt to reduce the amount of processor power in a mobile station. Continually searching for pilot signals and processing the results of pilot signal searches consumes processor power. The mobile station of the present invention indicates the search rate of secondary pilot signals may be changed based on the search rate of a reference pilot signal. The mobile station may also indicate an adjustment may be made to the amount of time spent searching for pilot signals by adjusting a variable length timer based on a determined safe search rate.